Article surveillance systems that work on the principle of detecting the presence of a particular form of ferromagnetic material in a periodically changing low level magnetic field are well known. U.S. Pat. No. 4,553,136 illustrates a representative system of this type in which an article to be protected against unauthorized removal from a defined area, such as a store or library, has a marker attached to it that has in it a strip of low coercivity ferromagnetic material of high permeability. When the article, with the marker attached, is brought into an interrogation zone in which the low level periodically changing magnetic field is applied via a radiating antenna located at the edge of the zone, the presence of the strip modifies the pattern of the applied field, generating magnetic sub-fields at harmonics of the fundamental frequency of the applied field. These harmonic fields are picked up by a receiving antenna also located at the edge of the interrogation zone and fed to a receiver where they are detected and used to trigger an alarm to signal the unauthorized removal of the article. Other representative examples of ferromagnetic marker systems of this general type are found in U.S. Pat. Nos. 3,665,449; 3,747,086; 3,820,104 and 4,710,752.
There is a recognized need for surveillance systems to provide corresponding security protection for computer disk magnetic media. With the proliferation in business and government of personal computers and the ready portability and easy concealability of the miniature data disks used by these computers it is important to provide article surveillance sensor systems for flexible computer data storage disks that ensure that sensitive data is not lost or compromised intentionally or unintentionally by unauthorized removal from secure areas in which the data is normally used. While non-magnetic marker systems are available for surveillance purposes and would have the advantage of not interfering with the magnetic media of data disks, they are generally not physically suited to the miniature size and slim geometries of conventional 5.25" and 3.5" flexible data disks employed with personal computers.
One proposal for a surveillance system for flexible magnetic data disks involves the attachment of a low coercivity, high magnetic permeability ferromagnetic marker strip to an inside surface of the jacket in which the disk is housed. This, however, would allow the system to be easily defeated by removing the disk from the jacket and reinserting it into another, non-protected jacket with the data on the disk still intact.
An improved form of surveillance protection for computer data disks is disclosed in the commonly assigned U.S. Pat. No. 4,910,625. As described in this patent, a band of preferably amorphous ferromagnetic marker material is integrally formed directly on the data disk about the central aperture of the disk and concentric therewith. This has the advantage that the disk itself bears the surveillance marker thus eliminating the ability to defeat the protection by removing the disk from the jacket. Moreover, by integrally forming the marker on the disk substrate, as by adhesive or ultrasonic bonding, any attempts to remove the marker will have a tendency to damage the disk thus effectively protecting the data on the disk from unauthorized removal from a secure area. While advantageous for this purpose, it has been found that extremely cautious removal of the marker, for example with the aid of a razor blade, can be effective, in some cases, to remove the marker without damaging the disk sufficiently to make the disk unusable, thus circumventing one of the purposes of integrally bonding the marker to the disk.
It is therefore an object of the present invention to provide improved article surveillance protection for a computer data disk, and more specifically for a flexible disk.